In the processing of fibers into textile structures, the fibers undergo a variety of stresses and strains which cause fiber breakage. Fibers with improved elongability are capable of better withstanding these stresses and strains. Moreover, fabrics which contain fibers having good elongability have the advantage of being stretchable and wrinkle resistant.
When carbon and graphitic fibers are produced from a stabilized acrylic precursor fiber, the extensibility or percent extension is typically in the range of elongation is about 1 25 to 1.9%, depending upon the heat treatment, the degree of carbonization or graphitization and modulus of the fiber. Typical linear graphitic or carbon fibers are produced by processing tows of from 1,000 to 320,000 filaments through a zone temperature furnace which heat treats the fiber in a temperature range of from about 300.degree. C. graduated rapidly up to about a 1050.degree. to 1100.degree. C. temperature range. This treatment is generally followed by a subsequent heat treatment in a high temperature furnace where the fiber is taken up at a temperature of from about 1400.degree. to 2400.degree. C. The heat treatment is carried out under tension even in a low temperature furnace. That is, the fibers are suspended through the furnace with sufficient tension to pull the fiber tows through the furnace and to keep them off the floor or bottom of the furnace.
It is especially advantageous if one desires to perform textile processing to have a percent elongation of from about 3 to 9% or greater. When a partially carbonized fiber, that is, a fiber which still have a nitrogen content of from 10 to 20%, is heat treated at from 550.degree. to 650.degree. C. under tension, the extensibility of the fiber is only 2.5% or less. This low extensibility is insufficient for textile processing without encountering considerable fiber breakage.
U.S. Pat. No. 4,347,297 to Mishima et al discloses a process for the preparation of carbon fibers by two preoxidation treatments of polyacrylonitrile fibers under tension and the carbonizing of the oxidized fibers under tension.
U.S. Pat. No. 4,279,612 to Saji et al discloses a method for producing carbon fibers which includes the step of thermally stabilizing the fibers under tension before heat treatment to carbonize the fibers.
U.S. Pat. No. 3,541,582 to Fainborough et al which discloses the preparation of woven carbon cloth by first oxidizing continuous yarns of polymeric fibers while under tension. The carbonization step is performed either while under tension or without tension. However, the woven cloth inherently places the fibers under tension.
U.S. Pat. No. 4,837,076 to Mc Cullough et al, which is herein incorporated by reference, discloses a process for preparing non-linear carbonaceous fibers, yarns and tows having a reversible deflection greater than 1.2:1. The conditions for heat treatment described in the patent can be used to provide similar electrical conductivity to the linear fibers of the invention. However, there is not disclosed any improvement in elongability other than as a result of the non-linear configuration. The non-linear carbonaceous fibers have a pseudoelongability as well as elongability after fiber tension. The non-linear fibers are prepared by heat treating a woven or knitted fabric and then deknitting. The use of a woven or knitted fabric adds costs to the process, and additionally provides a sufficient degree of stress to create cracks and voids.
U.S. Pat. No. 4,937,140, to Mc Cullough et al, which is herein incorporated by reference, discloses a process of fluorinating carbonaceous fibers which may be utilized in the present invention to provide fluorinated linear fibers having improved elongability.
U.S. Pat. No. 5,051,216 to Nakatani et al discloses a carbon fiber having an improved modulus of elasticity. The carbon fiber is prepared by applying tension to the fiber while heating. However, it has been found that applying heat to carbonaceous fibers under tension results in cracks and voids which weakens the fibers.
The term "carbonaceous fibers, tows and yarns" is understood to mean fibers, yarns and tows which have been heated to have an increased carbon content, namely, a carbon content of greater than 65% or an increase in carbon content as a result of an irreversible chemical reaction.
The term "graphitic" as used herein relates to those carbonaceous materials having an elemental carbon content of at least about 92%, preferably, about 98%, and as further defined in U.S. Pat. No. 4,005,183 to Singer, which is herewith incorporated by reference.
The term "pseudoelongability" as used herein relates to the percent elongation of a non-linear (crimped) fiber to remove the crimp so as to form a linear fiber without any internal molecular rearrangement of the fiber.
It is to be understood that the percentage stated relate to percent by weight of the total composition unless stated otherwise.